1. Field of the Invention
This invention relates to a process for producing an age-hardened copper titanium alloy strip. More particularly, it relates to the production of an age-hardened copper titanium alloy strip composed of a fine and uniform crystal structure and having excellent and uniform physical properties.
2. Description of the Prior Art
An age-hardened copper titanium alloy strip has excellent mechanical strength and electrical conductivity and is, therefore, used often to make electrically conductive springs of the thin plate type. Such spring sheets are usually produced by a process which includes preparing a copper titanium melt, casting it, hot-working the cast copper titanium, subjecting the hot-worked copper titanium to alternate annealing and cold working to a strip having a final shape. Then, the obtained strip is subjected to solution heat-treatment, and cold-worked again as required, and finally age-hardened.
The process employs, generally, a temperature of 800.degree. C. to 900.degree. C. for the recrystallization annealing or softening, or solution heat treatment of the alloy. This temperature is higher than a temperature of 720.degree. C. to 800.degree. C. which is usually employed for the recrystallization softening of an age-hardening copper beryllium alloy which is also used to make electrically conductive springs, and which is disclosed, for example, in U.S. Pat. No. 4,425,168 issued to Goldstein et al. on Jan. 10, 1984. The use of such a high temperature greatly promotes the growth of crystal grains, and the final product has an average crystal grain size of 40 microns or more, and sometimes even as large as 100 microns.
The coarsening of crystal grains adversely affects the formability, spring life, elongation, yield strength and other properties of the alloy and is a major cause for a wide range of variation in its properties. These problems associated with copper titanium alloy strips have hitherto been very difficult to solve, and no effective solution has been found.